Measuring memory wear and data retention individually based on cell voltage distributions

ABSTRACT

A memory system or flash card may include a mechanism for memory cell measurement and analysis that independently measures/predicts memory wear/endurance, data retention (DR), read disturb, and/or remaining margin. These effects may be independently quantified by analyzing the state distributions of the individual voltage levels of the cells. In particular, a histogram of cell voltage distributions of the memory cells can be analyzed to identify signatures for certain effects (e.g. wear, DR, read disturb, margin, etc.). Those measurements may be used for block cycling, data loss prediction, or adjustments to memory parameters. Pre-emptive action at the appropriate time based on the measurements may lead to improved memory management and data management. That action may include calculating the remaining useful life of data stored in memory, cycling blocks, predicting data loss, trade-off or dynamic adjustments of memory parameters.

PRIORITY AND RELATED APPLICATIONS

This application claims priority to Provisional patent applicationsentitled “MEASURING MEMORY WEAR AND DATA RETENTION INDIVIDUALLY BASED ONCELL VOLTAGE DISTRIBUTIONS” assigned Provisional Application Ser. No.62/095,608; “DYNAMIC PROGRAMMING ADJUSTMENTS IN MEMORY FOR NON-CRITICALOR LOW POWER MODE TASKS” assigned Provisional Application Ser. No.62/095,594; “TRADE-OFF ADJUSTMENTS OF MEMORY PARAMETERS BASED ON MEMORYWEAR OR DATA RETENTION” assigned Provisional Application Ser. No.62/095,633; “DYNAMIC PROGRAMMING ADJUSTMENTS BASED ON MEMORY WEAR,HEALTH, AND ENDURANCE” assigned Provisional Application Ser. No.62/095,612; “END OF LIFE PREDICTION BASED ON MEMORY WEAR” assignedProvisional Application Ser. No. 62/095,619; “MEMORY BLOCK CYCLING BASEDON MEMORY WEAR OR DATA RETENTION” assigned Provisional Application Ser.No. 62/095,623; “PREDICTING MEMORY DATA LOSS BASED ON TEMPERATUREACCELERATED STRESS TIME” assigned Provisional Application Ser. No.62/095,586; each of which were filed on Dec. 22, 2014 and each of whichis hereby incorporated by reference.

This application is further related to U.S. patent Ser. No. 14/977,174,entitled “END OF LIFE PREDICTION BASED ON MEMORY WEAR,” filed on Dec.21, 2015; U.S. patent Ser. No. 14/977,155, entitled “MEMORY BLOCKCYCLING BASED ON MEMORY WEAR OR DATA RETENTION,” filed on Dec. 21, 2015;U.S. patent Ser. No. 14/977,191, entitled “PREDICTING MEMORY DATA LOSSBASED ON TEMPERATURE ACCELERATED STRESS TIME,” filed on Dec. 21, 2015;U.S. patent Ser. No. 14/977,237, entitled “TRADE-OFF ADJUSTMENTS OFMEMORY PARAMETERS BASED ON MEMORY WEAR OR DATA RETENTION,” filed on Dec.21, 2015; U.S. patent Ser. No. 14/977,222, entitled “DYNAMIC PROGRAMMINGADJUSTMENTS BASED ON MEMORY WEAR, HEALTH, AND ENDURANCE,” filed on Dec.21, 2015; U.S. patent Ser. No. 14/977,227, entitled “DYNAMIC PROGRAMMINGADJUSTMENTS IN MEMORY FOR NON-CRITICAL OR LOW POWER MODE TASKS,” filedon Dec. 21, 2015; U.S. patent Ser. No. 14/977,144, entitled “FAILED BITCOUNT MEMORY ANALYTICS,” filed on Dec. 21, 2015; U.S. patent Ser. No.14/977,187, entitled “REMOVING READ DISTURB SIGNATURES FOR MEMORYANALYTICS,” filed on Dec. 21, 2015; and U.S. patent Ser. No. 14/977,198,entitled “END OF LIFE PREDICTION TO REDUCE RETENTION TRIGGEREDOPERATIONS,” filed on Dec. 21, 2015; the entire disclosure of each ishereby incorporated by reference.

TECHNICAL FIELD

This application relates generally to memory devices. More specifically,this application relates to the measurement of wear endurance, wearremaining, and data retention in non-volatile semiconductor flashmemory. Those measurements may be used for block cycling, data lossprediction, end of life prediction, or adjustments to memory parameters.

BACKGROUND

Non-volatile memory systems, such as flash memory, have been widelyadopted for use in consumer products. Flash memory may be found indifferent forms, for example in the form of a portable memory card thatcan be carried between host devices or as a solid state disk (SSD)embedded in a host device. As the non-volatile memory cell scales tosmaller dimensions with higher capacity per unit area, the cellendurance due to program and erase cycling, and disturbances (e.g. dueto either read or program) may become more prominent. The defect levelduring the silicon process may become elevated as the cell dimensionshrinks and process complexity increases. Likewise, time and temperaturemay hinder data retention (DR) in a memory device. Increased time and/ortemperature may cause a device to wear more quickly and/or lose data(i.e. data retention loss). Bit error rate (BER) may be used as anestimate for wear, DR, or remaining margin; however, BER is merely theresult of the problem and may not be an accurate predictor. Further,using BER does allow a distinction between memory wear and dataretention. For example, a high BER may be caused by any one of wear,read disturb errors, DR, or other memory errors.

SUMMARY

At any moment, the integrity of data in a block may be impacted by anycombination of wear, retention loss, read disturb or a presence of badcells. Being able to measure at any time and in any block, dataretention loss and rate independently from wear, read disturb and otherphenomena may provide improved memory analytics. In particular, it maybe desirable to independently measure/predict memory wear/endurance,data retention (DR), and/or remaining margin. The wear (wear endured andwear remaining), DR (retention capability and retention loss), andmargin remaining of memory cells may be independently quantified byanalyzing the state distributions of the individual voltage levels ofthe cells. Rather than relying on BER as an indicator, an independentmeasurement may be made for any of wear, endurance, DR, or read disturb.Pre-emptive action at the appropriate time based on the measurements maylead to improved memory management and data management. That action mayinclude calculating the remaining useful life of data stored in memory,cycling blocks, predicting data loss, trade-off or dynamic adjustmentsof memory parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a host connected with a memory systemhaving non-volatile memory.

FIG. 2 is a block diagram of an exemplary flash memory device controllerfor use in the system of FIG. 1.

FIG. 3 is a block diagram of an alternative memory communication system.

FIG. 4 is a block diagram of an exemplary memory system architecture.

FIG. 5 is a block diagram of another exemplary memory systemarchitecture.

FIG. 6 is a block diagram of an exemplary memory analysis process.

FIG. 7 is a block diagram of another exemplary memory analysis process.

FIG. 8 is a block diagram of a system for wear and retention analysis.

FIG. 9 is an example physical memory organization of the system of FIG.1.

FIG. 10 is an expanded view of a portion of the physical memory of FIG.4.

FIG. 11 is a diagram of exemplary super blocks.

FIG. 12 is a diagram illustrating charge levels in a multi-level cellmemory operated to store two bits of data in a memory cell.

FIG. 13 is a diagram illustrating charge levels in a multi-level cellmemory operated to store three bits of data in a memory cell.

FIG. 14 is an exemplary physical memory organization of a memory block.

FIG. 15 is an illustration of an exemplary three-dimensional (3D) memorystructure.

FIG. 16 is an exemplary illustration of errors due to read disturb,wear, and/or retention loss.

FIG. 17 is another exemplary illustration of errors due to read disturb,wear, and/or retention loss.

FIG. 18 is a histogram of exemplary cell voltage distribution states ina three bit memory wordline after the first program/erase cycle.

FIG. 19 is a cell voltage distribution illustrating location shift.

FIG. 20 is an expanded version of the G state cell voltage locationshift.

FIG. 21 is a cell voltage distribution illustrating distribution widthand shape changes.

FIG. 22 is an expanded version of the G state cell voltage distributionscale changes.

FIG. 23 is an expanded version of the G state cell voltage distributionshape changes.

FIG. 24 illustrates read disturb effects on voltage states with changesin the read threshold.

FIG. 25 illustrates a widening effect due to wear.

FIG. 26 illustrates the function for translating state widening tofailed bit count.

FIG. 27 illustrates data retention errors.

FIG. 28 illustrates state shift and retention time depending on theblock.

FIG. 29 illustrates an exemplary wear parameter.

FIG. 30 illustrates the end of life point for multiple bit error rate(BER) trajectories.

FIG. 31 illustrates different BER slopes that originate from the sameinitial BER value.

FIG. 32 illustrates an extension of FIG. 31 showing an average of BERslopes.

FIG. 33 illustrates calculating the BER can be used for a more accurateend of life calculation.

BRIEF DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The system described herein can independently quantize wear and dataretention. The quantization may be based on an analysis of the cellvoltage distribution or a bit error rate (BER) analysis. Changes to thecell voltage distribution or BER are analyzed to identify either wear ordata retention problems.

Data retention may refer to either a gain or loss of charge over time.Data may be lost if the charge gain/loss passes over a threshold voltagewhich then changes the value of the cell. An erase cycle may reset thecharge for the cells in a block, which can correct the gain/loss ofcharge over time. Read disturb errors may be caused when cells in amemory block change over time (e.g. become programmed unintentionally).It may be due to a particular cell being excessively read which maycause the read disturb error for neighboring cells. In particular, acell that is not being read, but receives elevated voltage stressbecause a neighboring cell is being read. Charge may collect on floatinggates, which may cause a cell to appear to be programmed. The readdisturb error may result in a data loss. ECC may correct the error andan erase cycle can reset the programming of the cell.

A retention capability may be predicted at any given program/erase (P/E)cycle and on any block, from a measurement of the wear and/or retentionloss rate of that block. DR predictions may be used for block leveling,recovering wasted margins, extending endurance, and for other productcapabilities. Periodic measurements of stored data can be used todynamically determine the wear or retention loss rates of individualblocks.

Memory wear refers to the finite limit of program-erase (P/E) cycles forthe memory. This may also be referred to as endurance. Memory may beable to withstand a threshold number of P/E cycles before memory weardeteriorates the memory blocks. A memory block that has failed shouldnot be used further. Wear leveling may be utilized as an attempt tonormalize P/E cycles across all blocks. This may prevent blocks fromreceiving excessive P/E cycles.

A flash memory system suitable for use in implementing aspects of theinvention is shown in FIGS. 1-5. A host system 100 of FIG. 1 stores datainto and retrieves data from a flash memory 102. The flash memory may beembedded within the host, such as in the form of a solid state disk(SSD) drive installed in a personal computer. Alternatively, the memory102 may be in the form of a flash memory card that is removablyconnected to the host through mating parts 104 and 106 of a mechanicaland electrical connector as illustrated in FIG. 1. A flash memoryconfigured for use as an internal or embedded SSD drive may look similarto the schematic of FIG. 1, with one difference being the location ofthe memory system 102 internal to the host. SSD drives may be in theform of discrete modules that are drop-in replacements for rotatingmagnetic disk drives. As described, flash memory may refer to the use ofa negated AND (NAND) cell that stores an electronic charge.

Examples of commercially available removable flash memory cards includethe CompactFlash (CF), the MultiMediaCard (MMC), Secure Digital (SD),miniSD, Memory Stick, SmartMedia, TransFlash, and microSD cards.Although each of these cards may have a unique mechanical and/orelectrical interface according to its standardized specifications, theflash memory system included in each may be similar. These cards are allavailable from SanDisk Corporation, assignee of the present application.SanDisk also provides a line of flash drives under its Cruzer trademark,which are hand held memory systems in small packages that have aUniversal Serial Bus (USB) plug for connecting with a host by plugginginto the host's USB receptacle. Each of these memory cards and flashdrives includes controllers that interface with the host and controloperation of the flash memory within them.

Host systems that may use SSDs, memory cards and flash drives are manyand varied. They include personal computers (PCs), such as desktop orlaptop and other portable computers, tablet computers, cellulartelephones, smartphones, personal digital assistants (PDAs), digitalstill cameras, digital movie cameras, and portable media players. Forportable memory card applications, a host may include a built-inreceptacle for one or more types of memory cards or flash drives, or ahost may require adapters into which a memory card is plugged. Thememory system may include its own memory controller and drivers butthere may also be some memory-only systems that are instead controlledby software executed by the host to which the memory is connected. Insome memory systems containing the controller, especially those embeddedwithin a host, the memory, controller and drivers are often formed on asingle integrated circuit chip. The host may communicate with the memorycard using any communication protocol such as but not limited to SecureDigital (SD) protocol, Memory Stick (MS) protocol and Universal SerialBus (USB) protocol.

The host system 100 of FIG. 1 may be viewed as having two major parts,insofar as the memory device 102 is concerned, made up of a combinationof circuitry and software. An applications portion 108 may interfacewith the memory device 102 through a file system module 114 and driver110. In a PC, for example, the applications portion 108 may include aprocessor 112 for running word processing, graphics, control or otherpopular application software. In a camera, cellular telephone that isprimarily dedicated to performing a single set of functions, theapplications portion 108 may be implemented in hardware for running thesoftware that operates the camera to take and store pictures, thecellular telephone to make and receive calls, and the like.

The memory system 102 of FIG. 1 may include non-volatile memory, such asflash memory 116, and a device controller 118 that both interfaces withthe host 100 to which the memory system 102 is connected for passingdata back and forth and controls the memory 116. The device controller118 may convert between logical addresses of data used by the host 100and physical addresses of the flash memory 116 during data programmingand reading. Functionally, the device controller 118 may include a Hostinterface module (HIM) 122 that interfaces with the host systemcontroller logic 110, and controller firmware module 124 forcoordinating with the host interface module 122, and flash interfacemodule 128. Flash management logic 126 may be part of the controllerfirmware 214 for internal memory management operations such as garbagecollection. One or more flash interface modules (FIMs) 128 may provide acommunication interface between the controller with the flash memory116.

A flash transformation layer (“FTL”) or media management layer (“MML”)may be integrated in the flash management 126 and may handle flasherrors and interfacing with the host. In particular, flash management126 is part of controller firmware 124 and MML may be a module in flashmanagement. The MML may be responsible for the internals of NANDmanagement. In particular, the MML may include instructions in thememory device firmware which translates writes from the host 100 intowrites to the flash memory 116. The MML may be needed because: 1) theflash memory may have limited endurance; 2) the flash memory 116 mayonly be written in multiples of pages; and/or 3) the flash memory 116may not be written unless it is erased as a block. The MML understandsthese potential limitations of the flash memory 116 which may not bevisible to the host 100. Accordingly, the MML attempts to translate thewrites from host 100 into writes into the flash memory 116. As describedbelow, an algorithm for measuring/predicting memory wear/endurance, dataretention (DR), and/or remaining margin (e.g. read disturb errors) mayalso be stored in the MML. That algorithm may analyze the statedistributions of the individual voltage levels of the cells, and utilizehistogram data of cell voltage distributions of the memory cells toidentify signatures for certain effects (e.g. wear, DR, margin, etc.).The flash memory 116 or other memory may be multi-level cell (MLC) orsingle-level cell (SLC) memory. MLC and SLC memory are further describedbelow. Either SLC or MLC may be included as part of the devicecontroller 118 rather than as part of the flash memory 116.

The device controller 118 may be implemented on a single integratedcircuit chip, such as an application specific integrated circuit (ASIC)such as shown in FIG. 2. The processor 206 of the device controller 118may be configured as a multi-thread processor capable of communicatingvia a memory interface 204 having I/O ports for each memory bank in theflash memory 116. The device controller 118 may include an internalclock 218. The processor 206 communicates with an error correction code(ECC) module 214, a RAM buffer 212, a host interface 216, and boot codeROM 210 via an internal data bus 202.

The host interface 216 may provide the data connection with the host.The memory interface 204 may be one or more FIMs 128 from FIG. 1. Thememory interface 204 allows the device controller 118 to communicatewith the flash memory 116. The RAM 212 may be a static random-accessmemory (SRAM). The ROM 210 may be used to initialize a memory system102, such as a flash memory device. The memory system 102 that isinitialized may be referred to as a card. The ROM 210 in FIG. 2 may be aregion of read only memory whose purpose is to provide boot code to theRAM for processing a program, such as the initialization and booting ofthe memory system 102. The ROM may be present in the ASIC rather thanthe flash memory chip.

FIG. 3 is a block diagram of an alternative memory communication system.The host system 100 is in communication with the memory system 102 asdiscussed with respect to FIG. 1. The memory system 102 includes a frontend 302 and a back end 306 coupled with the flash memory 116. In oneembodiment, the front end 302 and the back end 306 may be referred to asthe memory controller and may be part of the device controller 118. Thefront end 302 may logically include a Host Interface Module (HIM) 122and a HIM controller 304. The back end 306 may logically include a FlashInterface Module (FIM) 128 and a FIM controller 308. Accordingly, thecontroller 301 may be logically portioned into two modules, the HIMcontroller 304 and the FIM controller 308. The HIM 122 providesinterface functionality for the host device 100, and the FIM 128provides interface functionality for the flash memory 116. The FIMcontroller 308 may include the algorithms implementing the independentanalysis of wear and data retention as described below.

In operation, data is received from the HIM 122 by the HIM controller304 during a write operation of host device 100 on the memory system102. The HIM controller 304 may pass control of data received to the FIMcontroller 308, which may include the FTL discussed above. The FIMcontroller 308 may determine how the received data is to be written ontothe flash memory 116 optimally. The received data may be provided to theFIM 128 by the FIM controller 308 for writing data onto the flash memory116 based on the determination made by the FIM controller 308. Inparticular, depending on the categorization of the data it may bewritten differently (e.g. to MLC or retained in an update block).

FIG. 4 is a block diagram of an exemplary memory system architecture.The data storage system includes a front end 128, a flash transformationlayer (FTL) 126, and access to the NAND memory 116. The data storagesystem has its memory managed by the NAND memory management in oneembodiment. The NAND memory management may include a NAND trade-offengine 404, a block control module 406, and a memory analytics module408. The NAND trade-off engine 404 may dynamically measure deviceperformance and allow for adjustments to the device based on themeasurements. Power, performance, endurance, and/or data retention maybe emphasized or de-emphasized in the trade-off. For example, trimparameters may be adjusted based on the wear or data retention loss forcertain blocks. The trade-off may be automated for the device or it maybe adjusted by the user/host as described with respect to FIG. 5. Theblock control module 406 controls operations of the blocks. For example,the trim parameters that are adjusted may be individually adjusted foreach block based on the measurements of the block's health (e.g. wear,data retention, etc.), which is further described below. The memoryanalytics module 408 receives the individual health measurements forblocks or other units of the memory. This health of the blocks mayinclude the wear, data retention, endurance, etc. which may becalculated as described with respect to FIGS. 12-18. In particular, thememory analytics module 408 may utilize cell voltage distribution tocalculate the wear and the data retention independently for eachindividual block (or individual cells/wordlines/meta-blocks, etc.). Thearchitecture shown in FIG. 4 is merely exemplary and is not limited tothe use of a specific memory analytics implementation. Likewise, thearchitecture is not limited to NAND flash, which is merely exemplary.

FIG. 5 is a block diagram of another exemplary memory systemarchitecture. The system in FIG. 5 is similar to the system in FIG. 4,except of the addition of a memory analytics user interface 502. Thememory analytics user interface 502 may receive input from the user/host(through the front end 122) that is translated into system specifictrade-off bias. In particular, the memory analytics user interface 502may be user controlled by providing the user with an interface forselecting the particular trade-offs (e.g. low/high performance vs.high/low endurance or high/low data retention). In one embodiment, thememory analytics user interface 502 may be configured at factory and maybe one way to generate different product types (e.g. high performancecards vs. high endurance cards).

FIG. 6 is a block diagram of an exemplary memory analysis process. Thememory analytics 602 may include more precise measurements (includingvoltage and programming time) of the memory. For example, calculationand tracking of block level margins in data retention, endurance,performance, rates of change may be measured and tracked. That data canbe used for prediction of blocks' health towards end of life. The memoryanalytics may be performed by the memory analytics module 408 in oneembodiment. In one embodiment described below, the data retention(rate/loss) and the wear of individual blocks may be measured andtracked independently of one another.

Dynamic block management 604 may include leveling usage of blocks andhot/cold data mapping. This block management may be at the individualblock level and may include independent and dynamic setting of trimparameters as further discussed below. Further, the management mayinclude narrowing and recovering the margin distribution. The extramargins trade-offs 606 may include using recovered extra margins totrade off one aspect for another for additional benefits, and mayinclude shifting margin distributions. The trade-off product/interface608 may include configuring product type at production time, anddynamically detecting and taking advantage of idle time. This may allowa user to configure trade-offs (e.g. reduced performance for improvedendurance).

FIG. 7 is a block diagram of another exemplary memory analysis process.The process may be within the memory analytics module 408 in oneembodiment. Memory analytics may include an individual and independentanalysis of wear, data retention, read disturb sensitivity, and/orperformance. Each of these parameters may be measured and tracked(compared over periodic measurements). Based on the tracking, there maybe a prediction of certain values (e.g. overall endurance, end of life,data retention loss rate). Based on the predictions, certain functionsmay be performed, including block leveling or other system managementfunctions based on the individual values (e.g. wear or data retention).Adjustments can be made for dynamic block management based on thepredictions. Trade-offs (e.g. performance vs. endurance/retention) maybe automatically implemented (or implemented by the host) based on themeasurements and predictions. As described below, wear may be calculatedfor individual blocks and those values may be used for implementingcertain system processes (block cycling or leveling) and programming canbe adjusted dynamically based on those values.

FIG. 8 is a block diagram of a system for wear and retention analysis.In particular, FIG. 8 illustrates modules for performing the wear andretention analysis described below. A measurement module 802 or measurermay measure the cell voltages. For example, special read commands may beissued, such as those described below with respect to FIG. 18. The cellvoltage values can then be used generate a cell voltage distribution 806by the generation module 804 or generator. An exemplary cell voltagedistribution is shown below in FIGS. 12-13. There may be multiple cellvoltage distributions 806 that are compared by the comparison module 808or comparator. The cell voltage distributions may be periodicallygenerated and compared with each other, or compared with a referencecell voltage distribution that was generated when the memory was freshand new (e.g. at factory). In alternative embodiments, the absolutevalues of a cell voltage distribution may be used to estimate wear anddata retention of memory (without comparing other distributions). Ananalysis module 810 or analyzer may calculate or estimate wear and/ordata retention based on the cell voltage distribution. Based on the wearand/or data retention, the analysis module 810 may make furthercalculations discussed below, including but not limited to calculatingthe remaining useful life of data stored in memory, cycling blocks,predicting data loss, trade-off or dynamic adjustments of memoryparameters. In particular, modules such as a locator 812, scaler 814,and/or shaper 816 may analyze the cell voltage distribution as furtherdescribed with respect to FIG. 18. The locator 812 can determine dataretention based on a location shift of the states in the cell voltagedistribution as described with respect to FIG. 20. The scaler 814 maydetermine wear based on changes to the width of the states in the cellvoltage distribution as described below with respect to FIG. 22. Theshaper 816 may determine wear based on changes to the shape of thestates in the cell voltage distribution as described below with respectto FIG. 23.

The system may be implemented in many different ways. Each module, suchas the measurement module 802, the generation module 804, the comparisonmodule 806, and the analysis module 810, may be hardware or acombination of hardware and software. For example, each module mayinclude an application specific integrated circuit (ASIC), a FieldProgrammable Gate Array (FPGA), a circuit, a digital logic circuit, ananalog circuit, a combination of discrete circuits, gates, or any othertype of hardware or combination thereof. Alternatively or in addition,each module may include memory hardware, for example, that comprisesinstructions executable with the processor or other processor toimplement one or more of the features of the module. When any one of themodules includes the portion of the memory that comprises instructionsexecutable with the processor, the module may or may not include theprocessor. In some examples, each module may just be the portion of thememory or other physical memory that comprises instructions executablewith the processor or other processor to implement the features of thecorresponding module without the module including any other hardware.Because each module includes at least some hardware even when theincluded hardware comprises software, each module may be interchangeablyreferred to as a hardware module.

The data retention results or memory wear results from the cell voltagedistribution changes may be tracked and stored (e.g. in the flash memoryor within the controller). For example, a system table may track thechanges in the cell voltage distributions and resultant changes in dataretention and/or wear. By keeping an ongoing record of this information,a more accurate determination can be made regarding both wear and dataretention. This information may be used for optimizing short term andlong term storage of data. In particular, data that is not accessedfrequently (long term storage or “cold data”) should be stored wheredata retention is high. The variation in data retention may be block byblock or die by die.

In one embodiment, each comparison of a currently measured cell voltagedistribution may be compared with a reference cell voltage distribution(e.g. when the memory “fresh” such as at factory or at the first use).This reference cell voltage distribution is compared with each of thecell voltage distributions that are periodically measured such that arate at which the data is degrading in the cell can be determined. Thedeterminations that can be made from the calculations include:

-   -   Wear that a population of cells has endured;    -   Rate at which the population of cells is wearing;    -   Expected wear remaining of the population of cells;    -   Retention loss of the data stored in the cells;    -   Rate of retention loss of the data stored in the cells;    -   Margin to further retention loss can be determined; and    -   Retention loss rate may be used as a metric for retention        capability.

FIG. 9 conceptually illustrates an organization of the flash memory 116(FIG. 1) as a cell array. FIGS. 9-10 illustrate different sizes/groupsof blocks/cells that may be subject to the memory analytics describedherein. The flash memory 116 may include multiple memory cell arrayswhich are each separately controlled by a single or multiple memorycontrollers 118. Four planes or sub-arrays 902, 904, 906, and 908 ofmemory cells may be on a single integrated memory cell chip, on twochips (two of the planes on each chip) or on four separate chips. Thespecific arrangement is not important to the discussion below. Ofcourse, other numbers of planes, such as 1, 2, 8, 16 or more may existin a system. The planes are individually divided into groups of memorycells that form the minimum unit of erase, hereinafter referred to asblocks. Blocks of memory cells are shown in FIG. 9 by rectangles, suchas blocks 910, 912, 914, and 916, located in respective planes 902, 904,906, and 908. There can be any number of blocks in each plane.

The block of memory cells is the unit of erase, and the smallest numberof memory cells that are physically erasable together. For increasedparallelism, however, the blocks may be operated in larger metablockunits. One block from each plane is logically linked together to form ametablock. The four blocks 910, 912, 914, and 916 are shown to form onemetablock 918. All of the cells within a metablock are typically erasedtogether. The blocks used to form a metablock need not be restricted tothe same relative locations within their respective planes, as is shownin a second metablock 920 made up of blocks 922, 924, 926, and 928.Although it may be preferable to extend the metablocks across all of theplanes, for high system performance, the memory system can be operatedwith the ability to dynamically form metablocks of any or all of one,two or three blocks in different planes. This allows the size of themetablock to be more closely matched with the amount of data availablefor storage in one programming operation.

The individual blocks are in turn divided for operational purposes intopages of memory cells, as illustrated in FIG. 10. The organization maybe based on a different level (other than block or page level) includingat the word line level as further described below. The memory cells ofeach of the blocks 910, 912, 914, and 916, for example, are each dividedinto eight pages P0-P7. Alternatively, there may be 16, 32 or more pagesof memory cells within each block. The page is the unit of dataprogramming and reading within a block, containing the minimum amount ofdata that are programmed or read at one time. However, in order toincrease the memory system operational parallelism, such pages withintwo or more blocks may be logically linked into metapages. A metapage1002 is illustrated in FIG. 9, being formed of one physical page fromeach of the four blocks 910, 912, 914, and 916. The metapage 1002, forexample, includes the page P2 in each of the four blocks but the pagesof a metapage need not necessarily have the same relative positionwithin each of the blocks. A metapage may be the maximum unit ofprogramming.

FIGS. 9 and 10 are merely exemplary arrangements of pages. Theorganization of wordlines may be used rather than pages. Likewise, thesizes of pages (e.g. metapages) may vary for the memory analyticsdiscussed herein. In one embodiment, there may be flash super blocks.FIG. 11 illustrates flash super blocks and wordlines are furtherillustrated in FIGS. 14-15.

FIG. 11 illustrates an arrangement of super devices or super blocks.Super blocks may be similar to or the same as metablocks. Super blocksmay include erased blocks from different die (e.g. two erased blocksfrom different planes), accessed via a controller's NAND channels. Superblocks may be the smallest erasable unit in some cases. A super blockmay be broken into separate erased blocks which can be used toreconstruct a new one. For memory analytics, erased blocks may begrouped based on different characteristics to make a super block asuniform as possible. A super device may be a group of flash dies thatspans across all 16 channels as shown in FIG. 11. The flash dies thatform super devices may be fixed through the life of the drive. FIG. 11illustrates four super devices. In alternative embodiments, somecapacity drives may not have all four dies populated. Depending on thesize of the drive, fewer dies per channel may be populated. A superblock may be a group of erase blocks within a super device. Since thesuper block spans multiple channels, it may be concurrently writing tothe all die within a super block. With single-plane operations, each diemay contribute one erase block to a super block. As a result, each superblock may have the same number erase blocks as die within a super block.Advantages for using super blocks include fewer blocks to manage andinitialize. For example, instead of managing erase-block lists, thelists may cover super-block lists. Also, program/erase (PIE) counts andvalid-page counters may be managed at the super-block level. Anotheradvantage includes fewer metadata pages because each metadata pages in asuper block captures the metadata for multiple erase blocks. Withoutsuper blocks, each erase block would have a metadata page that used onlya fraction of the page. Super blocks may reduce the number of openblocks that are written to. For host writes there may be only fewersuper blocks for writing instead of a larger number of erase blocks.

FIG. 12 is a diagram illustrating charge levels in cell memory. Thecharge storage elements of the memory cells are most commonly conductivefloating gates but may alternatively be non-conductive dielectric chargetrapping material. Each cell or memory unit may store a certain numberof bits of data per cell. In FIG. 12, MLC memory may store four statesand can retain two bits of data: 00 or 01 and 10 or 11. Alternatively,MLC memory may store eight states for retaining three bits of data asshown in FIG. 4. In other embodiments, there may be a different numberof bits per cell.

The right side of FIG. 12 illustrates a memory cell that is operated tostore two bits of data. This memory scheme may be referred to as eX2memory because it has two bits per cell. The memory cells may beoperated to store two levels of charge so that a single bit of data isstored in each cell. This is typically referred to as a binary or singlelevel cell (SLC) memory. SLC memory may store two states: 0 or 1.Alternatively, the memory cells may be operated to store more than twodetectable levels of charge in each charge storage element or region,thereby to store more than one bit of data in each. This latterconfiguration is referred to as multi-level cell (MLC) memory. FIG. 12illustrates a two-bit per cell memory scheme in which either four states(Erase, A, B, C) or with two states of SLC memory. This two-bit per cellmemory (i.e. eX2) memory can operate as SLC or as four state MLC.Likewise, as described with respect to FIG. 4, three-bit per cell memory(i.e. eX3) can operate either as SLC or as eight state MLC. The NANDcircuitry may be configured for only a certain number of bit per cellMLC memory, but still operate as SLC. In other words, MLC memory canoperate as a MLC or SLC, but with regard to the MLC operation three bitper cell memory cannot operate as two bit per cell memory andvice-versa. The embodiments described below utilize any MLC memoryscheme's ability to work with SLC to then operate at different bits percell.

FIG. 12 illustrates one implementation of the four charge levels used torepresent two bits of data in a memory cell. In implementations of MLCmemory operated to store two bits of data in each memory cell, eachmemory cell is configured to store four levels of charge correspondingto values of “11,” “01,” “10,” and “00.” Each bit of the two bits ofdata may represent a page bit of a lower page or a page bit of an upperpage, where the lower page and upper page span across a series of memorycells sharing a common word line. Typically, the less significant bit ofthe two bits of data represents a page bit of a lower page and the moresignificant bit of the two bits of data represents a page bit of anupper page. The read margins are established for identifying each state.The three read margins (AR, BR, CR) delineate the four states. Likewise,there is a verify level (i.e. a voltage level) for establishing thelower bound for programming each state.

FIG. 12 is labeled as LM mode which may be referred to as lower atmiddle mode and will further be described below regarding the lower atmiddle or lower-middle intermediate state. The LM intermediate state mayalso be referred to as a lower page programmed stage. A value of “11”corresponds to an un-programmed state or erase state of the memory cell.When programming pulses are applied to the memory cell to program a pagebit of the lower page, the level of charge is increased to represent avalue of “10” corresponding to a programmed state of the page bit of thelower page. The lower page may be considered a logical concept thatrepresents a location on a multi-level cell (MLC). If the MLC is twobits per cell, a logical page may include all the least significant bitsof the cells on the wordline that are grouped together. In other words,the lower page is the least significant bits. For a page bit of an upperpage, when the page bit of the lower page is programmed (a value of“10”), programming pulses are applied to the memory cell for the pagebit of the upper page to increase the level of charge to correspond to avalue of “00” or “10” depending on the desired value of the page bit ofthe upper page. However, if the page bit of the lower page is notprogrammed such that the memory cell is in an un-programmed state (avalue of “11”), applying programming pulses to the memory cell toprogram the page bit of the upper page increases the level of charge torepresent a value of “01” corresponding to a programmed state of thepage bit of the upper page.

FIG. 13 is a diagram illustrating charge levels in a multi-level cellmemory operated to store three bits of data in a memory cell. FIG. 13illustrates MLC memory with three bits of data which are stored in asingle cell by establishing eight states or voltage level distinctions.This memory may be referred to as X3 memory. FIG. 13 illustrates thestages that may be used for programming three bit memory. In a firststage, the voltage levels are divided out at two levels, and at thesecond stage (i.e. foggy program), those two levels are divided up intothe eight states without setting the distinct levels between states. Atthe third stage (i.e. fine program), the voltage levels for each of theeight states are separated and distinct. The fine programmingestablishes the voltage levels for each of the states. As compared withtwo bit memory, the three bit memory in FIG. 13 requires more exactprogramming voltages to avoid errors. Electron movement or loss from thecharge values may result in problems. Endurance and programming speedmay decrease based on the exact programming that is required.

In alternative embodiments, there may be memory schemes with increasedbits per cell (e.g. 4 bits per cell or X4 memory). Each of those memoryschemes may operate using that number of bits per cell (e.g. “n” bitsper cell where n is an integer of 2 or more), but also by using SLCprogramming. Accordingly, the system and methods described herein willallow operation under n bits per cell or using SLC programming to actlike a different bit per cell memory (e.g. any number less than n).

The memory analytics described below captures data from analyzingmultiple states. For example, in FIG. 13, states (A through G) may beanalyzed. In one embodiment, the upper tail of the erase (Er) state (themain body of which is below 0V and may not be characterizable. Trackingmultiple states plus the Er tail may provide the best signal to noiseratio. The system could (for reasons of simplicity or to reduce theamount of data being tracked) track data from less than the full numberof states. In the case of the state shifting due to retention loss, themagnitude may be greater on the upper states. It might be moreadvantageous to simply track upper states or just one, for example, theG state (as shown in FIGS. 15-18). Further, FIG. 24 illustrates thestates with differing thresholds. The decision as to which state(s) totrack may be made according to which give the best signal of theparameter being tracked.

FIG. 14 is an illustration of an exemplary three-dimensional (3D) memorystructure. FIG. 14 illustrates an exemplary 3D NAND flash with Bit CostScaling (BiCS) using charge trapping memory. The source lines and bitlines are further described an illustrated with respect to FIG. 15. Theflash memory used in the storage system may be flash memory of 3D NANDarchitecture, where the programming is achieved through Fowler-Nordheimof the electron into the charge trapping layer (CTL). Erase may beachieved by using a hole injection into the CTL to neutralize theelectrons, via physical mechanism such as gate induced drain leakage(GIDL). FIG. 14 is an exemplary 3D structure with each cell beingrepresented by a memory transistor forming a memory column vertically(e.g., 48 wordlines). The wordlines (WL), bitlines (BL), and stringnumber are shown in FIG. 6. Four exemplary strings are shown. There maybe a memory hole (within a NAND column) that includes a memory holecontact. One exemplary wordline (logical wordline LWL 185) isillustrated along with an exemplary physical wordline (PWL 46).

FIG. 15 is an exemplary physical memory organization of a memory block.FIG. 15 illustrates a page of memory cells, organized for example in theNAND configuration, being sensed or programmed in parallel. A bank ofNAND chains are shown in the exemplary memory. A page may be any groupof memory cells enabled to be sensed or programmed in parallel. The pageis enabled by the control gates of the cells of the page connected incommon to a wordline and each cell accessible by a sensing circuitaccessible via a bit line (bit lines BL0-BLm). As an example, whenrespectively sensing or programming the page of cells, a sensing voltageor a programming voltage is respectively applied to a common word line(e.g. WL2) together with appropriate voltages on the bit lines. A silicagate drain (SGD) 1502 is shown opposite from a decoding gate, such assilica gate source (SGS) 1504. SGS 1504 may also be referred to as thesource gate or source, while SGD 1502 may be referred to as the draingate or drain. Word lines may be the unit by which memory analytics areperformed.

The memory analytics described herein may be utilized at differentlevels including at the block level, metablock level, super block level,die level, wordline level, page level, etc. The memory analyticsmeasurements and analysis may be described herein at the block level,but that is merely exemplary.

FIG. 16 is an exemplary illustration of errors due to read disturb,wear, and/or retention loss. Retention loss may be when the charge in acell is lost which causes a bit error by a change in value of the cell.As shown in the diagram, the retention loss increases over time. Thesignature of retention loss is a shift in the upper states. Wear is theexcessive usage of cells which may also result in errors. The signatureof wear is a skewing or widening of voltage states.

Read disturb errors may be caused when cells in a memory block changedue to interference from the reading of other cells in the vicinity. Itmay be due to a particular cell being excessively read which may causethe read disturb error for neighboring cells. In particular, a cell thatis not being read, but receives elevated voltage stress because aneighboring cell is being read. Charge may collect on floating gates,which may cause a cell to appear to be programmed. In alternativeembodiments, the memory may not use floating gates. For example, 3Dmemory may be a charge trap rather than a floating gate. The readdisturb error may result in a data loss. Read disturb is shown with anelevated bit count. The signature of read disturb is a widening of theerror (Er) state and possible widening of lower programmed states (e.g.A state).

FIG. 17 is another exemplary illustration of errors due to read disturb,wear, and/or retention loss. FIG. 17 is further described below withreference to the bit error rate (BER) process due to the overlappedstate but is applicable to the histogram process described herein. FIG.17 may be an illustration of data retention loss rate tracking.

The memory analytics described herein address each of these conditionsand account for them. In one embodiment, utilization of their respectivesignatures may be used for identification and measurement of individualcontributing factors that lead to data errors. In one embodiment, thesignatures of the read disturb, data retention, and wear may be usedwith a histogram analysis. In another embodiment, a bit error rate (BER)analysis of the slopes of the BER may be utilized for the memoryanalytics. The BER analysis is further described below with respect tothe Error Rate Based Tracking shown in FIGS. 25-29. The goal of thememory analytics may include a more complete understanding of the stateof the memory which may be achieved by looking at more discrete units ofthe memory (e.g. the block level or other levels).

Memory systems undergo write/erase operations due to both host writesand the memory maintenance operations in the normal life span of itsapplication. The internal memory maintenance (i.e. non-host writeoperations or background operations) can introduce a high writeamplification factor (“WAF”) for both MLC and SLC. WAF may be the amountof data a flash controller has to write in relation to the amount ofdata that the host controller wants to write (due to any internalcopying of data from one block to another block). In other words, WAF isthe ratio of non-host write operations compared with writes from thehost. In one example, up to half of the MLC write/erase operations maybe due to these internal memory operations. This may have a significanteffect on the life of the card. Accordingly, it may be important toreduce the endurance impact due to a system's internal write/eraseoperations.

Memory maintenance (which is interchangeably referred to as non-hostwrites and/or background operations) may be performed only at optimaltimes. One example of memory maintenance includes garbage collectionwhich may be needed to aggregate obsolete data together in blocks to beerased. Garbage collection can group together valid data and groupobsolete data. When a block includes only obsolete data, it can beerased so that new data can be written to that block. Garbage collectionis used to maximize storage in blocks by minimizing the number ofpartially used blocks. In other words, garbage collection may be aconsolidation or aggregation of valid data from blocks that have amixture valid data and obsolete data that results in more free blockssince there are fewer blocks that have a mixture of both valid andobsolete data. The background operations may further include themeasurement of cell voltages and/or the analysis of those voltages toindependently identify data retention or memory wear issues as discussedbelow.

FIG. 18 is a histogram of exemplary cell voltage distribution states ina three bit memory wordline after the first program/erase (P/E) cycle.There are eight states associated with three bit memory (X3). Differentmemory (X2 memory with two bits and four states) may be analyzedsimilarly to the example shown in FIG. 18. The distribution of thoseeight states is shown in FIG. 18 after the first P/E cycle. This rawdata may be collected by sending a set of sequences to the flash memorya “Distribution Read” sequence. The raw Distribution Read data is thenprocessed to produce a histogram of the voltage levels in all the cellsin the population. When the memory is described as having a certain wearor data retention loss, the reference to memory generally may refer tofinite portions of the memory, such as block level, groups of blocks(e.g. the groups described with respect to FIGS. 9-10), page, plane,die, or product level. An exemplary population to obtain a flash memoryunit (FMU), which may be statistically sufficient for the analysis andcalculation describe herein. The FMU may be the smallest data chunk thatthe host can use to read or write to the flash memory. Each page mayhave a certain number of FMUs.

Once the histogram is obtained, the individual state distributions maybe analyzed and characterized for: 1) Location; 2) Scale; and 3) Shape.For each of the eight states, the location, scale, and shape may bedetermined. A set of meta-data parameters (e.g. location, scale, shape)may be produced for the population. The meta-data may be used in eitherrelative or absolute computations to determine the wear and retentionproperties of the population.

Location may refer to the location of the distribution may include someform of a linear average, such as the mean or mode. As shown in FIG. 18,the location is determined with the mean in one embodiment. Location maybe calculated with other metrics in different embodiments.

Scale may include a measurement for the width of the distribution. Inone embodiment, scale may be measured by a deviation such as thestandard deviation, which is shown as sigma (σ) for each state. Inalternative embodiments, a percentile measurement may be used (e.g.width of 99% of values). Scale may be measured with other metrics thatquantify the width of the distribution in different embodiments.

Shape may include the skewness of the distribution. The skewness may bemeasured by asymmetry. In one embodiment, asymmetry may be determinedwith Pearson's Shape Parameter. Pearson's is merely one example ofasymmetry measurement and other examples are possible.

The controller 118 may include a measurement module that measures thecell voltage distribution for cells for generating a histogram such asthe example shown in FIG. 18. The controller may issue special readcommands to the flash memory. In particular, the special read commandsthat are used to generate the histogram are gradually moving from zerovolts up to a threshold voltage value. In other words, the controllersends special read commands to the NAND and the results are providedback to the controller. The special read command may a voltage signalthat is gradually increased (e.g. 0 to 6 Volts, increased by 0.025 Voltsfor each signal as in the example of FIG. 18). This may be referred toas ramp sensing. The results at the controller are those cells thatsensed to one. The initial measurement could be at manufacture and/orafter the first programming and results in the reference cell voltagedistribution that is used for comparing with subsequent measurements forquantifying the changes in distribution.

In the example of FIG. 18, the voltage value is gradually increased fromzero volts to above six volts with a step size of 0.025 volts. Thevoltage is increased by 0.025 volts for each step and the number ofcells that are changed in value (e.g. sensed from zero to one) ismeasured for the histogram. Starting at zero volts, all the programcells are above zero, so the result at zero is a frequency of zero.Moving up a step (e.g. 0.025 volts or another voltage step), the cellsare again read. Eventually, there is a voltage threshold (e.g. as partof the A state) where there are cells that are programmed at thatvoltage. At any given cell threshold voltage (x-axis of the histogram)certain cells are sensed and that frequency is measured (y-axis of thehistogram). Each value for the cell threshold voltage may be viewed as abin of voltage values. For example at 0.6 Volts, the frequency beingshown is really those cells that are sensed between 0.6 V and 0.625 V(where the step size is 0.025 V). The difference between cells below(value of 0=below) at 0.6 V from cells above at 0.625 V is thefrequency. In other words, the voltage distribution may be thedistribution of cells within a step size (e.g. 25 mV steps) that weretriggered above the higher value of the step size (minus the cellstriggered at the lower value of the step size).

The absolute values from the histogram may be used for identifyingparameters (e.g. wear, data retention, etc.). Alternatively, thehistogram generation may occur periodically and the relative positionsfor the histogram may be used for identifying those parameters. In oneembodiment, the periodic measurements may be based on timing (e.g.hours, days, weeks, etc.) or may be based on events (e.g. duringbackground operations). FIG. 21 (described below) illustrates wideningdue to wear. Although not shown, the histogram may change after more P/Ecycles. FIGS. 7-8 illustrate the cell voltage distribution of the 8states (A-G) of the 3-bit (X3) memory. In alternative embodiments, theremay be more or fewer states depending on the memory. The distributioncalculations described herein can apply to a memory with any number ofstates.

FIG. 19 is a cell voltage distribution illustrating distribution shift.FIG. 19 illustrates one distribution with no bake time (0 hour baketime) and one distribution after being baked for ten hours (10 hour baketime). The baking process includes exposing the memory to a very hightemperature over a short time to simulate exposure at a normaltemperature over a much longer time. Over time, data may be lost fromthe memory (even at normal temperatures) and the baking provides amechanism for testing this data loss in a shorter amount of time (e.g.10 hours of bake time rather than years of time at a normaltemperature). Even at normal temperatures, electrons may leak from thefloating gates over time, but the baking process just speeds up thatleakage for testing purposes.

FIG. 19 illustrates that the data loss (i.e. poor data retention)results in a gradual shift of the distribution. In particular, FIG. 19is an illustration of analysis of data retention (DR). The right mostdistributions (i.e. the E, F, and G distributions) have a downward(lower voltage) shift due to the lapse in time (simulated by bake time).In the embodiment of FIG. 19, this is performed with a minimal amount ofP/E cycles (indicated as 0Cyc in the legend) so that wear will notinfluence the calculations. In other words, the memory wear is isolatedfrom the data retention parameter because only fresh blocks are beingbaked. The result is a distribution that has no change to scale orshape, but does have a location change. Accordingly, a location shift ofthe distribution is indicative of a data retention problem.

Upper State Tracking

Upper state tracking may be a subset of the previous embodiments or itmay be a separate method used for tracking all states or for readdisturb (RD) signature removal. In one embodiment, the tracking of anupper state may be used for memory analytics. This analysis may be partof an analysis of cell voltage distribution. In particular, the upperstate tracking may utilize only an upper state for the memory analyticswhere a cell voltage distribution of one or more of the upper states maybe representative of the memory as a whole. For example, referring toFIG. 13, the G state may be used for this purpose and the tracking maybe referred to as G state tracking. FIGS. 20-23 may include anillustration of G state cell voltage distribution.

FIG. 20 is an expanded version of the G state cell voltage distributionshift. In particular, FIG. 10 illustrates the G state (the highestvoltage state) from FIG. 19 with a normalized y-axis (frequency maximumsfrom FIG. 19 are normalized by peak value to one). The two lines shownare one with no bake time (0 Hr) and a distribution after a ten hourbake time (10 Hr). The distribution shift is more clearly shown in FIG.20 and may be referred to as the location. The location may becalculated as the difference in the shift of the modes between the twodistributions or the difference in the shift of the means between thetwo distributions. In this embodiment, only the G state is examinedbecause the largest (and easiest to measure) shift occurs in the Gstate. In alternative embodiments, the shifts of any combination of theother states may also be measured and used for calculating dataretention problems. For example, shifts from different states could becombined and the average or gradient information for those shifts may beanalyzed. The gradient of the relative shifts of different distributionsmay provide information for the location.

While a shift of the cell voltage distribution may be indicative of dataretention, a change in shape of the cell voltage distribution may beindicative of wear. FIG. 21 is a cell voltage distribution illustratingdistribution scale and shape changes. FIG. 21 illustrates a distributionwith limited usage (0 Cyc=no/limited P/E cycles) and a distribution withhigh usage (2000 Cyc=2000 P/E cycles). Unlike in FIGS. 19-20 there is nobake time (simulating elapsed time) for this distribution because itonly illustrates changes caused by P/E cycles. FIG. 21 illustrates thatthe both the scale/width and shape of the distribution are changed bywear. In other words, the scale/width and shape change of a distributionare indicative of wear. FIG. 22 describes using cell voltagedistribution width for determining wear and FIG. 23 describes using cellvoltage distribution shape for determining wear.

FIG. 22 is an expanded version of the G state cell voltage distributionscale changes. Wear results in a widening of the scale of thedistribution. Accordingly, a quantification of the shape widening can beindicative of wear. In one embodiment, the width may be quantified usingthe standard deviation of the distribution. Alternatively, percentilesof the scale may also be used. For example, FIG. 22 illustrates (withthe dotted line widths) an exemplary 50% point on the distribution and adetermination may be made as to where it crosses the x-axis. In otherwords, a comparison of the lengths of the two dotted lines in FIG. 22 isan exemplary value for the scale/width.

FIG. 23 is an expanded version of the G state cell voltage distributionshape changes. As an alternative to scale/width measurements of thechanges to the distribution, the shape/asymmetry/skewness of thedistribution may be analyzed for the wear analysis. As discussed,Pearson's Shape Parameter is one exemplary way to measure asymmetry. Theshape changes to the distribution as a result of wear may modify thedistribution as shown in FIG. 23. The G-state can be used to exclude theRD component, instead of using RD margin for end of life (EOL) and/orother estimates. Other states (e.g. E or F), or combinations of states,may be more representative for wear and DR measurements. G-state ismerely one example for this measurement.

As with FIG. 20, both FIG. 22 and FIG. 23 are normalized with therespect to the y-axis based on each distribution's respective peakvalue. Since only the voltage value (x-axis) matters for thequantization of any of the location, scale, or shape, the y-axis valuesdo not matter. Accordingly, the normalization of the y-axis does notaffect the voltage values, and does not affect the quantization of thelocation, scale, and shape.

Wear and retention loss are independent variables using this cellvoltage distribution analysis. In particular, an analysis of the cellvoltage distribution of the memory can be used to independently quantizewear, or may be used to independently quantize retention loss. Increasedwear does not affect retention loss, and retention loss does not affectwear. In other words, when cells wear, the cell voltage distributionwidens and changes shape, but the location does not change. Likewise,when data retention worsens, the cell voltage distribution shiftslocation, but the width and shape of the distribution do not change.Merely determining BER as an indicator of either wear or retention lossdoes not allow for identifying either parameter independently. However,a determination of BER with read thresholds may be used to measure shiftand widening, as indicators for wear and/or DR. This determination isfurther described below. Skew may be hard to measure but can beapproximated using pre-measured data.

The measurements and generation of the histogram values may be acontroller intensive process that is run only as a background operationto minimize performance issues for the user. In one embodiment, themeasurement and collection of the histogram data may be stored inhardware, such as in firmware of the device. Likewise, hardware may alsoperform the analyzing (e.g. calculation and comparison of location,scale, shape, etc.) of the histogram described herein. There may be acomponent or module (e.g. in the controller or coupled with thecontroller) that monitors the distribution changes (location shifts, andwidth or shape changes) of the cell voltage distribution to identify orpredict data retention or wear problems. In one embodiment, this may bepart of a scan that is specific for either data retention loss or wear.Alternatively, the scan may be associated with a garbage collectionoperation. A periodic measurement of the cell voltage distribution canbe made and stored. That data may be periodically analyzed to identifywear (using either width or shape distribution changes) or retentionloss (using location distribution changes).

End of Life Prediction Based on Memory Wear

The data loss (retention) and/or memory wear that are independentlydetermined may be used for predicting the life remaining in the system.The end of live (EOL) prediction may be based on the memory analyticsusing histograms above. Alternatively, the EOL prediction may be basedon the bit error rate (BER) method described below.

System life may be predicted by the lifetime of the worst X blocks inthe system. X may be the number of spare blocks required for operation.If the wear remaining of all blocks in the system is ordered from lowestwear remaining to highest wear remaining, then system life may bepredicted by the wear remaining of the Xth ordered block. The Xthordered block may be the measure for the system life because when allthe blocks up to and including this block are retired, then the systemmay cease functioning Specifically, if there are no spare blocksremaining, then the system may transition to read only mode and may notaccept new data.

In one embodiment, FIG. 17 may be an end-of-life calculation. Block 1704may measure DR loss utilizing memory analytics. In block 1706, DR lossrate is calculated and in block 1708, the current BER is estimated atend-of-retention margin. In block 1710, the end-of-retention predictionis updated along with the block's maximum P/E value.

The system life calculation may be utilized with any method whichcalculates wear remaining of individual blocks. As described above, thewear remaining is calculated independently by analysis of the cellvoltage distribution. Other embodiments, may calculate wear remaining ofthe individual blocks through other methods. The system life may stillbe estimated based on the wear remaining of the block that is the Xthmost worn, where X is total number of spare blocks required.Accordingly, the independent calculation of wear remaining discussedabove may merely be one embodiment utilized for this calculation ofoverall system life.

The data loss (retention) and/or memory wear that are independentlydetermined may be used for determining which blocks to select forreclamation and subsequent use for new host data. As discussed above,hot count may not be an accurate reflection of true wear on a block.Cycling blocks using the actual wear remaining calculated for each ofthe blocks may be more accurate. The system endurance may be extended tothe average wear remaining of all blocks in the system. This increasessystem endurance over the system endurance that relies on hot count wearleveling. The blocks are cycled in an attempt to level the wearremaining for each block. In particular, blocks with the lowest wearremaining may be avoided, while blocks with the most wear remaining maybe utilized in order to normalize the wear remaining. This wear levelingmay extend the life of the device by avoiding the blocks with the leastwear remaining, which prevents them from going bad and being unusable.

A calculation of actual wear remaining for each block allows for eachblock to be leveled based on actual wear rather than based on the hotcount (which may not reflect actual wear remaining). The actual wear maybe the error rate or bit error rate. Further, program/erase (P/E)failure probability may be a symptom of actual wear. Measuring wear rate(which may define BER rate due to P/E cycles) may be better than using alarge margin assuming every block is as bad as the worst one in thepopulation. In other words, the worst may be the one with the leastnumber of P/E cycles before maximum BER at maximum retention. This mayalso apply to DR loss rate. Any method for individually determining thewear remaining for individual blocks may be utilized for this wearleveling, including the calculation of wear remaining by analysis of thecell voltage distribution described above. More accurate wear levelingincreases overall system endurance because the system endurance becomesthe average capability of all blocks in the system.

Data loss prediction can be improved by predicting or estimating elapsedtime and/or temperature changes. Charge may dissipate over time or athigher temperatures, resulting in a potential data loss if a cellcrosses a threshold. Predicting when this may occur can allow for datato be scheduled to be refreshed before it is lost, but not so frequentlythat it would cause unnecessary wear. Knowing the retention timeremaining for the data in each block in the system can be used foridentifying which blocks are in need of being refreshed as compared withother blocks and can be used for identifying which blocks must berefreshed in order to avoid a loss of the data. Previous approaches mayhave used assumptions for rate loss that is based on a worst casescenario. Having the data loss prediction or data retention informationfor each block allows for a more accurate estimate of overall data lossand more efficiency in refreshing blocks.

Retention loss rate may be measured by making periodic measurements ofthe cell voltage distribution as described above, and computing the rateof change in units common to all blocks in the system. Once an accurateretention loss rate is determined for all blocks in the system, thezero-time retention capability of all blocks can be computed. Blocks canthen be retired or used for purposes other than long-term data retentionbased on their retention capability (e.g. if retention capability fallsbelow that value required to meet warranty requirements). At any time,the retention life remaining of all data stored in the device may becompared and provided in response to a system query. This may be usefulfor archival situations where the device is periodically powered up andthe life remaining of the data stored within the device must beaccurately known.

Memory Block Cycling

The memory block cycling may be a wear leveling method based on the EOLprediction described above. The data loss (retention) and/or memory wearthat are independently determined may be used for determining whichblocks to select for reclamation and subsequent use for new host data.Cycling blocks using the actual data loss (retention rate/margin)calculated for each of the blocks may be more accurate than relying onhot count for block cycling. The system endurance and retentioncapability may be extended to the average retention margin remaining ofall blocks in the system. The blocks are cycled in an attempt to preventdata loss for each block. In particular, blocks with the lowest dataretention levels or data retention rates may be selected for reclamationand subsequent use, while blocks with the best data retention may notneed to be cycled/refreshed. This may normalize the data retention ratesof all blocks. This cycling of blocks may extend the life of the deviceby refreshing blocks with data retention issues, or even cycling out anyblocks with poor data retention that cannot be fixed with refreshing. Inone embodiment, blocks with a higher data retention rate may be used forlonger term data, while blocks with a lower data retention rate may beused for shorter term data. This may be to divert data traffic whichtriggers more garbage collections and P/E cycles, to those blocks whichcan handle more P/E cycles. Also, it helps to reduce DR triggeredcopies, as described below. Likewise, the blocks with a higher dataretention rate may be used for more important data. For example, thebetter blocks may be used for control information and address tables.

A calculation of data retention for each block allows for each block tobe cycled based on actual data retention rather than based on the hotcount (which may not reflect actual data retention). Any method forindividually determining the data retention for individual blocks may beutilized for this cycling, including the calculation of data retentionby analysis of the cell voltage distribution described above. Moreaccurate data retention cycling increases overall system data retentioncapability because the system data retention capability becomes theaverage capability of all blocks in the system.

Optimization between performance and endurance may be achieved usingvalues for memory wear value from each of the blocks. The program speed(or harshness), which as an example can be effected by the width in timeof programming pulses and the voltage magnitude of programming pulses,for each block may be set to achieve a specific wear rate (endurancecapability). The higher the program speed, the more the memory cellswill be worn but the program time will be lower leading to higherperformance. In this way there a fundamental programmable trade-offbetween wear and performance that can be achieved. By combining rawmeasurements of block performance capability (time to erase/time toprogram) with the wear or retention of each block, the program rate foreach block can be set optimally which results in a distribution ofprogram times that are individually tuned for each block to maximize theendurance for a given minimum performance. For example, a lower programrate provides decreased performance, but increased endurance. Likewise,a higher program (programming faster) provides better performance, butrisks reduced endurance/lifetime. Because the wear and data retentionare known for individual blocks, the program rate for those blocks maybe independently modified. In other words, the optimization may be madeon a block-by-block basis. Blocks with high wear may be programmedslower than blocks with low wear. Likewise, blocks with poor dataretention may be programmed slower than blocks with good data retention.

Endurance may be maximized to increase the device lifetime.Alternatively, performance may be maximized for all blocks in the systemto satisfy a given minimum block endurance requirement. Thisperformance/endurance optimization may be made and adjusted during runtime of the device. In particular, the wear rate and data retention foreach block can be updated periodically, so those updated values can beused to update the optimization. In one embodiment, the user may adjustthe trade-off between performance and endurance. This dynamic adjustmentthat optimizes between performance and endurance, which results in amore customizable device.

An accurate measurement of the data retention loss (i.e. temperatureaccelerated stress time) may be made due to time/temperature while adevice was switched off. The precise temperature accelerated stress timeof the power-off period is predicted and may be used to re-compute theage of all data in the system. Upon power up, the data retention loss(i.e. retention margin) may be re-measured for each block. The valuesfor data retention loss may be compared to the trend predicted byprevious measurements. As described above, the data retention (or wear)for individual blocks may be periodically measured and a rate of changemay be calculated. This change or trend may be compared with the valuesafter power up. Changes to the trend may be due to a long power-offperiod or higher temperature during the power-off period and may have acumulative negative effect on the device. The effective temperatureaccelerated stress time during a power-off period may be computed basedon the trend changes. Accurate temperature accelerated stress timeestimates can be used to re-compute the age or retention life remainingof all data stored in the system. Changes to the wear or data retentionbetween power off and power on can be used to estimate the temperatureaccelerated stress time for any power off period based on changes to thewear and/or data retention upon power up, after that power off period.Knowledge of the wear and/or data retention for each individual blockmay allow for a more accurate estimate of temperature accelerated stresstime than would otherwise be estimated using BER. Because the changes inthose values are periodically measured, all systems that rely on suchdata will have up to date information and corresponding actions can betaken.

Applications such as wear levelling and prediction of end of life orcertain error rate in the future may all be temperature dependant.Therefore, the predictions and estimates may be temperature dependent.For example, if end of life is determined by the event of reachingmaximum number of errors after the minimum retention period, the n^(th)worst case scenario should be considered for the temperature mode. Themeasurements of data retention loss rate may also be dependent ontemperature accelerated time, rather than absolute time. Effects ofwear, DR, and RD as measured (e.g. number of errors) may depend on thecurrent temperature. The measurements of absolute values, change ratesand predictions as explained in the examples only makes sense if thetemperature is the same. If it is not, then the correction has to bemade to adjust the values to the current temperature (for absolutemeasurements) and to the temperature accelerated time (for change rate),and adjusted for specific temperature mode (for predictions).

NAND Flash memory may traditionally utilize static trim parameters,using the same programming mode for the same product. A trim parametermay include one or more parameters related to read operations, includinga program rate, a program voltage level, a step-up voltage or step size,and/or a program pulse width. For example, the trim settings may includea sensing time or sense amplifier delay, and/or a sensing or sensereference voltage. The initial setting of the trim parameters may be setup for the fastest and most aggressive programming mode possible withinthe endurance requirements for the worst block. However, a memory testat production may require extensive testing to make sure that all blocksmarked as good meet the performance and endurance criteria. By utilizingindependent measurements of wear and/or data retention rate for eachindividual block, the identification of good or bad blocks using trimparameters may be dynamic and may be more accurate. In particular, theindividual measurements of data retention for each block may be tracked(i.e. current values compared with initial values of data retention).Combined with program and erase (P/E) time measurements, temperatureaccelerated stress time measurements, and block endurance estimates,outlier (potentially bad) blocks may be detected as having unacceptableperformance or data retention values (either based on a current value orbased on a predicted value using the tracked values). The detectedblocks may then be mapped out as bad if they are below a threshold. Thethreshold may be based on the health of the other blocks (e.g. thresholdmust be X % of average health) or may be based on outlier blocks (healthdeviation from an average). Not only can this be performed on the blocklevel, but it may also be performed on the word-line level.

By setting trim parameters statically (e.g. at manufacture), there maybe unused margin in performance, endurance, and data retention. Dynamicblock management (e.g. 604 in FIG. 6) may include leveling the usage ofblocks and hot/cold data mapping, or modifying trim parametersindependently and dynamically, and at the block level. The managementmay include narrowing and recovering the margin distribution and theextra margins trade-offs (e.g. 606 in FIG. 6) may include usingrecovered extra margins to trade off one aspect for another foradditional benefits. A user may be able configure trade-offs, such asreduced performance for improved endurance.

Trade-offs that take advantage of unused, wasted margins of individualblocks may be made by the host and/or user. A host protocol may be setup externally with the trade-off bias. For example, there may bedifferent use cases for the host/user to choose between (e.g. high/lowpower/performance, low/high endurance, low/high data retention). Forexample, in FIG. 5, the memory analytics user interface 502 may receiveinput from the host (through the front end 128) that is translated intosystem specific trade-off bias. The trade-off can be changed atproduction or during life via the host's interface. Examples ofhost/user controlled trade-off conditions (i.e. over-clocking) mayinclude: 1) high-performance, standard endurance and retention; 2)high-performance, low endurance or/and retention; and/or 3) lowerperformance/power, high endurance and/or retention. These exemplaryoptions may be selected dynamically by the user, or may be set atproduction in the factory.

Dynamically throttling down programming parameters to make programmingmore gentle may cause less wear, but at the cost of programmingperformance. This dynamic throttling may be utilized when a high levelof wear is detected. Based on the measurements discuss above, wear maybe calculated for individual blocks or other units of the memory. Thehigh level of wear may be a threshold above which the memory is notdesigned to operate properly. The threshold may be set below thiscritical value at which a block becomes unusable. Performance throttlingmay then be triggered to extend endurance. Further, the trim parametersmay be dynamically changed. As discussed above, the trim parameters mayinclude one or more parameters related to read operations, including aprogram voltage, a step-up voltage, and/or a program pulse width. Forexample, higher endurance programming mode may be achieved by loweringthe programming voltage with finer programming pulses. Likewise, for ahigher data retention programming mode (in addition to lower wear mode),extra time may be sacrificed to allow a finer programming mode which canmake voltage distributions tighter and margins wider. Tighterprogramming with wider margins may cost performance but improve dataretention.

As with the dynamic throttling based on wear, the performance may alsobe throttled for a low power mode. A low power mode may also be a lowerperformance mode that is established by the device and/or host. In oneembodiment, the detection includes receiving a host's command to go tolow power mode, which allows for an operation at a lower speed.Alternatively, the device may detect a low battery level andautomatically trigger the low power mode. In yet another alternativeembodiment, a high temperature level may be detected which may requirethrottling down power to reduce heat dissipation. Accordingly, adetection of a lower power mode may be a signal to throttle performance(e.g. adjustment of trim parameters). For example, lower powerprogramming mode may be achieved by lowering the programming voltagewith finer programming pulses. Higher endurance programming mode orhigher data retention performance mode may both utilize lower power thana higher performance mode.

Devices in normal use have frequent idle times which can be used for GCwork by storage devices. Tasks during idle time may not be timecritical, so modern devices utilize user idle time to undertakebackground work that they immediately suspend once the user becomesactive. Such background work may compete against the device's need toperform pending GC work by sending commands to the storage deviceforcing it into a non-idle state. Reducing power consumption for thedevice while also increasing endurance can be achieved with the goal ofhaving sufficient time for necessary background operations.Identification of when a command is due to a user idle backgroundprocesses may allow the device to optimize itself to maximize enduranceand reduce power use.

Programming may be adjusted dynamically for tasks which are not timecritical. Just as there may be dynamic throttling for low power mode,there may also be throttling for tasks which are not time critical. Theidentification of a task which is not time critical may includedetecting on the drive or sub-drive/bank level or it may be a host'striggered background or idle mode, or detection of an inactive part of adrive. It may be detected on a die level, and individual die may be idleif there is no pending host writes. In this example, a background task,such as Garbage Collection (GC), may be implemented with lowerperformance. Various trim parameters (discussed above) may bedynamically adjusted for tasks that are not time critical.

Exemplary non-critical tasks may include: 1) tasks in which there was nohost command (e.g. background operations); 2) a command from the hostthat is identified as non-critical (e.g. iNAND products with commandsfrom the operating system); or 3) through the identification of a lowpriority period. The identification of a low priority period may beidentified by distinguishing between non-urgent “Low Priority CommandPeriod” host activity and urgent “High Priority Command Period” hostactivity. By distinguishing between these, the endurance may beincreased while also reducing power consumption of a device bydistinguishing between active user time and background operating andfile system commands sent to the device. Because low priority periodsare identified separate from high priority periods, the performance maynot suffer for this optimization. The following inputs may be used inorder to identify low priority command periods:

-   -   Rate of read sectors from the device over time;    -   Rate of written sectors to the device over time;    -   Data rate (reads and writes) as a proportion of maximum drive        data rate ability;    -   The time gap between commands being received from the host;    -   Pattern of writes to file system specific areas (e.g. NTFS        recovery zones); and    -   Changes in depth of the device's Native Command Queue (NCQ).

Patterns in the rate of work performed by the device may be analyzed toidentifier whether a particular task is not critical. For example, adevice may be busy, but the data pushed/pulled may be low, so despitebeing busy, this may be a non-time critical activity (idle time) sincethe read/write activity is low. In particular, the read/write (R/W) datarate over time may used to identify idle time. A pattern of low datarate corresponds to a low priority command period. In other words, whenthe data rate is low it may identify an idle time regardless of how busythe device may be.

There may be a threshold value for the data rate per period of time. Ifthe threshold value is exceeded, then the current period is not lowpriority. The threshold may be extended to longer or shorter periods formore accurate measurements (e.g. data rate per minute vs. data rate persecond). If the threshold value is exceeded, then the data rate may beexamined over a longer time period. In an alternative embodiment, theremay be a native command queue. When the host commands queue is backedup, this indicates a higher priority time period. For example, this maytrigger coming out of low priority mode.

Data rate may be used to identify a low priority command period (idletime) and non-critical tasks. Low data rate periods may be ideal timesto undertake GC work. When low priority command periods are detected,the device may be optimized by:

-   -   Remaining in garbage collection mode even when new commands        arrive from the host;    -   Having the ability to over-ride read priority during background        work detected mode (speed of reads may be considered less        important than getting garbage collection work completed);    -   Programming data more slowly to improve endurance;    -   Running transfer buses in low power mode (slower data rate);    -   Powering down dies and routing data to a single die (reduce        parallelism); and    -   Reducing RAM use and powering down banks of RAM to reduce power        use.

Error Rate Based Tracking

Wear can be identified and measured by its signature skew and wideningof the program states. Similarly, data retention changes have asignature of upper states shifting down in voltage (left on thedistribution diagram diagram). Also, the RD effect can be recognized.All three effects, wear, DR and RD contribute to read errors, asillustrated in FIG. 16. All three effects may be measured independently.

Characterizing the state histogram(s) for location, scale and shape andtracking this data over programming cycles and time may not be the onlyway to determine the amount of wear, retention loss or read disturb aunit of memory has experienced. Other methods may reduce the amount ofhistogram analysis by substituting inferred data from what is happeningto the error rate under certain conditions. For example, to determinethe amount that the erase state tail has shifted into the A-state (toseparate read disturb), it may be possible to map the data after it hasbeen corrected by the ECC engine from the state it was detected in tothe state that it should have been.

The proposed failed bit count (“FBC”) or error rate method may be basedon the error rate measurement, approximated by taking multiple reads andmeasuring the FBC and by using the optimal read thresholds. FBC, as asingle measure for a codeword or a page, may be extrapolated to theerror rate for a wordline and block by taking multiple measurements ofdifferent pages and wordlines, in order to represent a large block andremove the noise.

The FBC process may include 1) RD FBC—which is equal to zero at zeroretention time, as there were no reads; 2) DR FBC—also equal to zero atzero retention time, as there is no retention; and 3) Wear FBC—which isequal to total FBC at zero retention time, provided that the errors dueto bad cells are not counted. The bad cells may be identified andcounted at the beginning of life as static bad bits and removed from theanalysis as they may not indicate error changes die to wear. In otherwords, Total FBC=Wear FBC (as additional total FBC@(retentiontime=0)−bad cell error count)+DR FBC (=additional FBC @time=current)+RDFBC (=additional FBC @time=current). This may allow for the separationof FBC due to wear versus FBC due to DR, provided RD errors are removedas described below for the error-rate RD signature removal.

FIG. 24 illustrates read disturb effects on voltage states with changesin the read threshold. In particular, FIG. 24 is a simplified version ofoverlaps, including the Er and A state overlap. In addition, there is anA to B shift due to read disturb (RD). Read disturb may prevent anaccurate measurement of age or predicted lifetime of memory.Accordingly, an accurate assessment of lifetime may require aconsideration of potential read disturb effects. In particular, the readdisturb can shift the voltage states.

By counting the number of cells that were detected in the A-state but infact were part of the erase state, the scale of the erase statedistribution can be approximated. FIG. 24 illustrates the states beforereads. With read disturb, the states shift. The middle diagramillustrates the default read threshold, which is the read thresholdbefore any reads. Upon read disturb, the default read threshold may nolonger match the shifted data as can be seen in the middle diagram wherethe default read threshold does not match the data. Optimizing the readthreshold may include shifting the read threshold to an optimal valuebased on the shifted states. The optimal value of the read threshold maybe the value at which the bit error rate (BER) is minimized. As shown inthe diagrams, the area of the overlapped portion (diagonal lines) isminimized for the optimal read threshold. The overlapped portion may bequantified as the number of bit errors. The diagonally shaded area inthe bottom diagram shows errors due to Er state widening to A state, andmay not include errors due to A widening to Er state. In other words,the tail of the A state is left of the read threshold. The errors due toEr and A state overlap are made of two parts: 1) Er state overlap overthe read threshold, so it is read as A state (LP bit flips from 1 to0)—those errors are shown as diagonally shaded area; and 2) a stateoverlap over the read threshold, so it is read as Er state (LP bit flipsfrom 0 to 1). Although those errors are not shown on the Figures, theywould be on the area on the left of the read threshold line, as on thebottom diagram of FIG. 24. On the middle diagram, it does not exist, soin this example, there would be no overlap, as the read threshold is toolow. The optimal threshold voltage is selected to minimise the totalnumber of errors from 1) and 2) listed above.

The bottom diagram in FIG. 24 illustrates that the optimized readthreshold is shifted to be between the A state and the Erase state. Inalternative embodiments, each of the thresholds between each state maybe optimized. Alternatively, a representative read threshold may beanalyzed and movement of that threshold may be used to determine acorresponding movement of read thresholds for other states. Althoughthis method minimizes number of errors overall and the amount of thestate shift indicates severity of RD, it may be difficult to accuratelyseparate errors which happened due to RD from the errors which are theresult of Er-A and A-B overlaps due to wear and DR.

Removing Read Disturb Signatures

The use of bit error rates (BER) may replace the histogram analysisdescribed above. In particular, the BER memory analysis may be used forread disturb (RD) signature removal. Separate measurements of BER may beused for Er, A, B states (i.e. read disturb sensitive) and higher states(states C . . . G). Also, the proposed method may allow for thedistinguishing of Er state to A state errors from A state to Er stateerrors which may be part of the same overlap area. For example, one mayuse default read thresholds and one may use optimized read thresholds.Combined with the BER analysis the overlaps may be distinguished.Regardless of whether the read threshold is optimized, the BER may beused to measure data retention loss or read disturb. Another option maybe to remove Er, A, B states from the overall BER analysis of wear andDR rates. An extreme case may be to use only the G-state. Overall BERmay be approximated, as a more accurate indicator in a real drive thathas minimal RD noise.

To determine an amount that the erase state tail has shifted into theA-state (to separate read disturb), the data may be mapped (after beingcorrected by the ECC engine) from the state it was detected in to thestate that it should have been in. This quantization may be of theoverlap shown in FIG. 24. By counting the number of cells that weredetected in the A-state but in fact were part of the erase state, thescale of the erase state distribution can be approximated. For example,the LP 1 to 0 errors are where MP & UP=1 in the 2-3-2 encoding shown inthe diagram.

Referring back to FIG. 17 which may include the bit error rate (BER) dueto the overlapped state (left and right, can be measured and tracked).FIG. 17 may be an illustration of data retention loss rate tracking.Block 1702 may be a histogram capture with ramp sensing. Block 1704 maymeasure DR loss utilizing memory analytics, including a state histogramanalysis or via an alternative analysis with separate measurements ofBER due to Er, A, B states (RD sensitive) and higher states (C . . . G).The BER memory analysis is further described herein. The histogramanalysis was described above. In block 1706, DR loss rate is calculatedutilizing the current and previous measurements which may factor intemperature-accelerated stress time. In block 1708, the current BER isestimated at end-of-retention margin. In block 1710, theend-of-retention prediction is updated along with the block's maximumP/E value for wear leveling by utilizing the tracked block's wear rate(which may be orthogonal to DR loss rate).

In other words, the process may include: 1) a histogram capture (e.gramp sensing); 2) measuring a data retention loss utilizing memoryanalytics via separate measurements of BER due to Er, A, B states (RDsensitive) and higher states (C . . . G) to remove RD component; 3)calculate DR loss rate utilizing the current and previous measurements(which may factor in temperature-accelerated stress time); 4) estimatethe current BER at the end-of-retention margin which is furtherdescribed below; and 5) update the end-of-retention prediction and theblock's maximum P/E value for wear leveling by utilizing the trackedblock's wear rate (which may be orthogonal to the DR loss rate). Step 2)using the BER may differ from the embodiments discussed above in whichthe state histograms are analyzed to measure DR loss. In particular,rather than analyzing a histogram of the voltage state, the BER can bequantized (e.g. the actual number of errors between any two of thestates) and used for the analysis. The quantization may include left vs.right overlaps which can be separated. The histogram process describedabove is merely one embodiment, while the BER method is an alternativeembodiment. The BER method may be used to 1) remove RD; 2) track DRchanges; and 3) track wear changes.

The diagram in FIG. 24 may be used to identify which errors were localdistribution errors or which were from read disturb. The errors fromread disturb may be removed by processing the decoded data post-ECCcorrection such that the errors that were due to Er to A state flips,for example, can be removed from the FBC analysis before themeasurements of data retention or wear are made. This method would workfor either the default read threshold or the optimized read threshold,as shown in FIG. 24. The table shown in FIG. 24 is showingcorrespondence between upper, middle, and lower page bits. Inparticular, it illustrates the correspondence between the states andpages as shown in the chart.

With RD error removed, wear can be measured by measuring additional(since beginning of life) errors at zero retention time. Each block mayhave different wear rate, as widening of the states, and the resultingerror rate may not be a linear function as the errors are results of thestates' overlaps.

FIG. 25 illustrates a widening effect due to wear. FIG. 25 alsoillustrates the difference in overlaps between the states. Overlap 2502shows errors due to F state misread as G state, which results in someUpper Page bits read as 1 instead of 0. The other side of the overlapwould be Upper Page bits misread as 0 instead of 1. The bottom diagramshows the maximum number of P/E cycles with a larger overlap 2504. Theupper page (UP), middle page (MP), and lower page (LP) values are shownfor each distribution.

The widening effect, which may be measured using different parameters,such as standard deviation, per the histogram method, increases theoverlap area size (which is the total number of errors). The function ofhow wear FBC, or the overlap area, grows with more Program/Erase (P/E)cycles can be used to predict FBC for a certain number of P/E cyclesbased on at least two parameters: 1) the wear FBC delta, as block'scurrent FBC versus ‘fresh’ FBC (as measured at P/E=0); and 2) the numberof P/E cycles. The function to translate the state widening to FBC (orarea size), using the above parameters, may be a formula based or tablebased.

FIG. 26 illustrates the function for translating state widening to FBC.The function may allow for the prediction wear, in FBC terms, at certainnumber of P/E cycles. This function may be temperature dependent. As theblocks can be different qualities, “weak” blocks can develop widerstates faster than “strong” blocks. Although not illustrated, there maybe a skew in both FIGS. 25-26.

Data retention errors may be due to the state overlaps or upper stateshift. FIG. 27 illustrates data retention errors. The shifting of the Gstate is shown in FIG. 27 with different values. The first diagramillustrates 0 P/E cycles, minimum data retention, and no read disturb(RD). The second diagram illustrates 0 P/E cycles, maximum dataretention, and no RD. The third diagram illustrates the maximum numberof P/E cycles, minimum data retention, and no RD. The fourth diagramillustrates the maximum number of P/E cycles, maximum data retention,and no RD. The upper page (UP), middle page (MP), and lower page (LP)values are shown for each distribution. FIG. 27 may be similar to thediagrams shown in FIGS. 19-20 except it uses a logarithmic scale.

The shift may be measured directly by finding the optimal readthreshold, and then used as a parameter indicating DR loss, in order tocalculate DR loss rate. The shift may differ from block to block. FIG.28 illustrates state shift and retention time depending on the block.Strong blocks have a longer retention time than weak blocks.

The predicted shift (or predicted FBC due to shift) may be used tocalculate DR loss rate and predict FBC growth due to DR using afunction. The function may be a formula based or table based. It alsocan also be temperature dependent. DR loss also depends on block wear(worn out blocks have a faster voltage shift and the states themselvesare wider), so the overall function may be based cycles and based on atleast three parameters: 1) read voltage shift during the retentionperiod, or alternatively, block's DR FBC delta, as current FBC versuszero time FBC (same number of P/E cycles); 2) a block's retention time,to measure DR rate; and 3) a block's wear parameter. FIG. 29 illustratesan exemplary wear parameter. FIG. 29 shows different wear (low/high). InFIG. 29, weak may relate to DR only, and may or may not correlate to ablock assumed weak in the aspect of wear.

The combination of wear rate prediction and DR loss rate predictions maybe used together to predict FBC at certain retention time and at certainnumber of P/E cycles. This prediction may be used in differentapplications such as retention based levelling, where all blocks areused to have the maximum FBC at the minimum retention period.

Retention Trigger

Endurance may be maximized by managing the cycling of cells such thatthey all ultimately provide the same end of retention error rate at theend of life point. It may be more efficient to minimize data retentiontraffic and wear in the storage device in some cases such as coldstorage. If the underlying retention capability of memory units is nowknown, this information can be used to predict accurately when datastored within blocks needs to be recycled for retention managementpurposes, and left untouched otherwise. This may minimize the amount ofsystem traffic being generated for retention management which can be aperformance and endurance hit (e.g. in high capacity, cold storage, lowendurance memory applications). Retention management may be based on theworst case expected retention capability of all blocks in the system.This may be referred to as the baseline method. If there is someindication of the expected life (or temperature) of the data beingstored in the memory unit (e.g. from a detection method or from hosthints) (aka ‘cold data’), data with an expected long life (i.e. lowtemperature) may be directed to the memory units with the greatestretention capability. Likewise, data with an expected lower life may bedirected to memory units with the worst data retention capability.

Blocks may be refreshed at the maximum allowed retention period, equalto minimum retention requirement for the memory. The analysis may bebased on the worst block in the population to guarantee no data loss dueto retention. A refresh may include a copy of data to another block,resulting in additional P/E cycle and delay. In order to reduce wear andprevent performance loss, the retention trigger may include measuringretention rate and predicting the maximum BER. The block is refreshedonly when necessary and unnecessary P/E cycles and delays are minimized.This may be used for high capacity, cold storage, low endurance memorydrives, with high retention triggered data traffic and cycling. The endof life prediction may be used to reduce retention triggered operationssuch as block refreshment or rewriting operations. There may be a bruteforce method to measure BER and trigger DR refresh copy when BER isclose to the maximum. This method may not be practical due to the needto scan a significant amount of memory frequently (patrol). It mayrequire more margin, data traffic, and/or power. Also, it may be notpossible to do it all the time if the drive is powered off. Tracking DRrate may be more effective.

Referring back to FIG. 16, the slope of the bit error rate may bechanged based on read disturb. This change in slope prevents an accuratedetermination of end of life. Accounting for read disturb can allow fora more accurate trajectory of the bit error rate to further identify theend of life point. FIG. 30 illustrates the end of life point formultiple bit error rate (BER) trajectories. Accordingly, an estimate canbe made for end of life, but due to the effects of read disturb thiscalculation may be incorrect.

FIG. 31 illustrates different BER slopes that originate from the sameinitial BER value. In particular, the two dotted lines have the sameinitial BER value but the slopes are different which changes the end oflife prediction time. Accordingly, determining end of life requires theanalysis of read disturb and/or data retention discussed above foraccurately determining the slope of the BER.

FIG. 32 illustrates an extension of FIG. 31 with different BER slopes.The end of life point 3202 may be when the maximum error rate (see FIG.31) is met with the minimum retention (see FIG. 31). Two examples areshown in FIG. 32 with different BER slopes. Slope 3206 has a higher BERand reaches the maximum error rate in less time than the slope 3208which has a lower BER and reaches the maximum error rate in more time.By utilizing the slopes of all (or a representative number) of blocks,the maximum BER point may be predicted. An average curve 3204 may becalculated by the prediction of the point when the different blocksreach the maximum error rate. Curve 3204 may only illustrate thedistribution. It may be calculated to estimate the overall benefit ofthe method. In the device itself, it may be used to estimate time leftlimited by DR triggered refresh copies if the host never accesses thedrive. FIG. 32 illustrates an exemplary average curve 3204 thatillustrates a range of time when the maximum error rate is likely to bemet.

FIG. 33 illustrates calculating the BER can be used for a more accurateend of life calculation. Additional reads may accelerate overall errorrate so DR on its own either cannot be used, or should include RDmargin. In particular, by measuring each block's data retention lossrate and/or read disturb signature, the BER can be predicted at end oflife. Accordingly, blocks can be leveled so that the BER of each blockcan be directed towards the same end of life point.

Determining the slope of the BER for each block can be used to predictthe end of life for that particular block. Wear leveling can be used sothat the BER slope for each block is extended towards the same end oflife point as shown in FIG. 33.

Semiconductor memory devices include volatile memory devices, such asdynamic random access memory (“DRAM”) or static random access memory(“SRAM”) devices, non-volatile memory devices, such as resistive randomaccess memory (“ReRAM”), electrically erasable programmable read onlymemory (“EEPROM”), flash memory (which can also be considered a subsetof EEPROM), ferroelectric random access memory (“FRAM”), and magnetoresistive random access memory (“MRAM”), and other semiconductorelements capable of storing information. Each type of memory device mayhave different configurations. For example, flash memory devices may beconfigured in a NAND or a NOR configuration.

The memory devices can be formed from passive and/or active elements, inany combinations. By way of non-limiting example, passive semiconductormemory elements include ReRAM device elements, which in some embodimentsinclude a resistivity switching storage element, such as an anti-fuse,phase change material, etc., and optionally a steering element, such asa diode, etc. Further by way of non-limiting example, activesemiconductor memory elements include EEPROM and flash memory deviceelements, which in some embodiments include elements containing a chargestorage region, such as a floating gate, conductive nanoparticles, or acharge storage dielectric material.

Multiple memory elements may be configured so that they are connected inseries or so that each element is individually accessible. By way ofnon-limiting example, flash memory devices in a NAND configuration (NANDmemory) typically contain memory elements connected in series. A NANDmemory array may be configured so that the array is composed of multiplestrings of memory in which a string is composed of multiple memoryelements sharing a single bit line and accessed as a group.Alternatively, memory elements may be configured so that each element isindividually accessible, e.g., a NOR memory array. NAND and NOR memoryconfigurations are exemplary, and memory elements may be otherwiseconfigured.

The semiconductor memory elements located within and/or over a substratemay be arranged in two or three dimensions, such as a two dimensionalmemory structure or a three dimensional memory structure.

In a two dimensional memory structure, the semiconductor memory elementsare arranged in a single plane or a single memory device level.Typically, in a two dimensional memory structure, memory elements arearranged in a plane (e.g., in an x-z direction plane) which extendssubstantially parallel to a major surface of a substrate that supportsthe memory elements. The substrate may be a wafer over or in which thelayer of the memory elements are formed or it may be a carrier substratewhich is attached to the memory elements after they are formed. As anon-limiting example, the substrate may include a semiconductor such assilicon.

The memory elements may be arranged in the single memory device level inan ordered array, such as in a plurality of rows and/or columns.However, the memory elements may be arrayed in non-regular ornon-orthogonal configurations. The memory elements may each have two ormore electrodes or contact lines, such as bit lines and word lines.

A three dimensional memory array is arranged so that memory elementsoccupy multiple planes or multiple memory device levels, thereby forminga structure in three dimensions (i.e., in the x, y and z directions,where the y direction is substantially perpendicular and the x and zdirections are substantially parallel to the major surface of thesubstrate).

As a non-limiting example, a three dimensional memory structure may bevertically arranged as a stack of multiple two dimensional memory devicelevels. As another non-limiting example, a three dimensional memoryarray may be arranged as multiple vertical columns (e.g., columnsextending substantially perpendicular to the major surface of thesubstrate, i.e., in the y direction) with each column having multiplememory elements in each column. The columns may be arranged in a twodimensional configuration, e.g., in an x-z plane, resulting in a threedimensional arrangement of memory elements with elements on multiplevertically stacked memory planes. Other configurations of memoryelements in three dimensions can also constitute a three dimensionalmemory array.

By way of non-limiting example, in a three dimensional NAND memoryarray, the memory elements may be coupled together to form a NAND stringwithin a single horizontal (e.g., x-z) memory device levels.Alternatively, the memory elements may be coupled together to form avertical NAND string that traverses across multiple horizontal memorydevice levels. Other three dimensional configurations can be envisionedwherein some NAND strings contain memory elements in a single memorylevel while other strings contain memory elements which span throughmultiple memory levels. Three dimensional memory arrays may also bedesigned in a NOR configuration and in a ReRAM configuration.

Typically, in a monolithic three dimensional memory array, one or morememory device levels are formed above a single substrate. Optionally,the monolithic three dimensional memory array may also have one or morememory layers at least partially within the single substrate. As anon-limiting example, the substrate may include a semiconductor such assilicon. In a monolithic three dimensional array, the layersconstituting each memory device level of the array are typically formedon the layers of the underlying memory device levels of the array.However, layers of adjacent memory device levels of a monolithic threedimensional memory array may be shared or have intervening layersbetween memory device levels.

Then again, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device having multiplelayers of memory. For example, non-monolithic stacked memories can beconstructed by forming memory levels on separate substrates and thenstacking the memory levels atop each other. The substrates may bethinned or removed from the memory device levels before stacking, but asthe memory device levels are initially formed over separate substrates,the resulting memory arrays are not monolithic three dimensional memoryarrays. Further, multiple two dimensional memory arrays or threedimensional memory arrays (monolithic or non-monolithic) may be formedon separate chips and then packaged together to form a stacked-chipmemory device.

Associated circuitry is typically required for operation of the memoryelements and for communication with the memory elements. As non-limitingexamples, memory devices may have circuitry used for controlling anddriving memory elements to accomplish functions such as programming andreading. This associated circuitry may be on the same substrate as thememory elements and/or on a separate substrate. For example, acontroller for memory read-write operations may be located on a separatecontroller chip and/or on the same substrate as the memory elements.

One of skill in the art will recognize that this invention is notlimited to the two dimensional and three dimensional exemplarystructures described but cover all relevant memory structures within thespirit and scope of the invention as described herein and as understoodby one of skill in the art.

A “computer-readable medium,” “machine readable medium,”“propagated-signal” medium, and/or “signal-bearing medium” may compriseany device that includes, stores, communicates, propagates, ortransports software for use by or in connection with an instructionexecutable system, apparatus, or device. The machine-readable medium mayselectively be, but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. A non-exhaustive list of examples of amachine-readable medium would include: an electrical connection“electronic” having one or more wires, a portable magnetic or opticaldisk, a volatile memory such as a Random Access Memory “RAM”, aRead-Only Memory “ROM”, an Erasable Programmable Read-Only Memory (EPROMor Flash memory), or an optical fiber. A machine-readable medium mayalso include a tangible medium upon which software is printed, as thesoftware may be electronically stored as an image or in another format(e.g., through an optical scan), then compiled, and/or interpreted orotherwise processed. The processed medium may then be stored in acomputer and/or machine memory.

In an alternative embodiment, dedicated hardware implementations, suchas application specific integrated circuits, programmable logic arraysand other hardware devices, can be constructed to implement one or moreof the methods described herein. Applications that may include theapparatus and systems of various embodiments can broadly include avariety of electronic and computer systems. One or more embodimentsdescribed herein may implement functions using two or more specificinterconnected hardware modules or devices with related control and datasignals that can be communicated between and through the modules, or asportions of an application-specific integrated circuit. Accordingly, thepresent system encompasses software, firmware, and hardwareimplementations.

The illustrations of the embodiments described herein are intended toprovide a general understanding of the structure of the variousembodiments. The illustrations are not intended to serve as a completedescription of all of the elements and features of apparatus and systemsthat utilize the structures or methods described herein. Many otherembodiments may be apparent to those of skill in the art upon reviewingthe disclosure. Other embodiments may be utilized and derived from thedisclosure, such that structural and logical substitutions and changesmay be made without departing from the scope of the disclosure.Additionally, the illustrations are merely representational and may notbe drawn to scale. Certain proportions within the illustrations may beexaggerated, while other proportions may be minimized. Accordingly, thedisclosure and the figures are to be regarded as illustrative ratherthan restrictive.

We claim:
 1. A method for memory cell analysis in a memory devicecomprising: periodically determining a cell voltage distribution ofcells in the memory device during run time; measuring changes in thecell voltage distribution from the periodic determinations; calculatinga wear of the cells based on a shape change from the measured changes inthe cell voltage distribution; and calculating a data retention value ofthe cell based on a location change in the cell voltage distribution,wherein the wear and data retention value are independent variables suchthat the calculating of the wear and the calculating of the dataretention do not affect one another.
 2. The method of claim 1 whereinthe cell voltage distribution comprises a histogram.
 3. The method ofclaim 2 wherein the shape change comprises a measurement of skewness ofthe cell voltage distribution.
 4. The method of claim 3 wherein theskewness is calculated using Pearson's shape parameter.
 5. The method ofclaim 2 wherein the location change comprises a measurement of anaverage of the cell voltage distribution.
 6. The method of claim 1wherein the cell voltage distribution is normalized with respect to ahighest value on a y-axis and the changes measured are based on changesto an x-axis of the cell voltage distribution.
 7. The method of claim 1wherein the memory device comprises a non-volatile storage comprisingmemory blocks that include the memory cells.
 8. The method of claim 6wherein a controller is associated with operation of the memory blocks.9. The method of claim 8 wherein the controller performs the memoryanalysis.
 10. A method for measuring data retention comprising:performing the following in a storage module: measuring a voltagedistribution of an upper state for each cell in a memory; calculating alocation of the voltage distribution over time; and quantizing a dataretention based on a change in the calculated location.
 11. The methodof claim 10 wherein the location comprises a calculation of at least oneof a mean, a mode, or a median of the voltage distribution.
 12. A methodfor measuring wear comprising: performing the following in a storagemodule: measuring, periodically, voltage distribution of an upper statefor each cell in a memory; calculating a width of the distribution overa certain number of cycles; and quantizing wear based on changes to thewidth.
 13. The method of claim 12 wherein the width comprises acalculation of a standard deviation of the voltage distribution.